Factors Controlling the Membrane Efficiency of Shales When Interacting with Water-Based and Oil-Based Muds
- Jianguo Zhang (BP America) | Talal M. Al-Bazali (Kuwait U.) | Martin E. Chenevert (U. of Texas at Austin) | Mukul Mani Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2008
- Document Type
- Journal Paper
- 150 - 158
- 2008. Society of Petroleum Engineers
- 6.5.4 Naturally Occurring Radioactive Materials, 1.14 Casing and Cementing, 2.7.1 Completion Fluids, 5.1 Reservoir Characterisation, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.1 Hydrates
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This paper presents a comprehensive set of experimental data for the membrane efficiency of four shales when interacting with different water-based and oil-based muds. Pressure transmission tests were used to measure the membrane efficiency using three different cations and two different anions at different concentrations (water activities).
It was found that the measured membrane efficiencies of shales, when exposed to salt solutions, were low—ranging from 0.18% to 4.23%. Useful correlations are presented between the membrane efficiency and other shale properties. Results suggest that the membrane efficiency of shales is directly proportional to the ratio of the cation exchange capacity and permeability of shales. Higher cation exchange capacities and lower permeabilities correlate very well with higher membrane efficiencies. Moreover, the ratio of the hydrated solute (ion) size to shale pore throat determines a shale's ability to restrict solutes from entering the pore space and controls its membrane efficiency. Cations and anions with large hydrated radii yielded higher membrane efficiencies, compared to ions with small hydrated diameters. Thus, the formulation of drilling fluids must take into account the types of cation and anion in the water-based fluid.
It was also found that the membrane efficiency of oil-based muds was high, however, these membrane efficiencies were not 100 % as postulated by many researchers.
Background and Past Work
Shale interaction with drilling fluids differs from sandstone and carbonates interactions caused by the absence of mud cake formation, and that is attributed to the fact that the shale permeability is so low. Unlike drilling sandstones and carbonates, mud cakes, which normally act as semipermeable membranes do not form while drilling shales because the shale permeability is much lower than the mud cake permeability. Therefore, it is widely accepted that the shale itself could act as a semipermeable membrane sustaining osmotic flow.
Osmosis has long been recognized as a means to extract water out of a shale when the water activity of the shale is higher than that of the drilling fluid. In the absence of a hydraulic pressure gradient, the movement of mud filtrate into shale is mainly governed by the chemical potential difference between the pore fluid and the mud, and this results in the osmotic transport of water, (Ewy and Stankovich 2000). However, it has recently been shown that the osmotic potential generated between shale and drilling fluid is greatly influenced by the flow of ions into or out of shale caused by the ionic concentration imbalances (Zhang et al. 2004). Therefore, the actual osmotic effect is often less than the osmotic potential. This has spurred much interest to quantify the impact of ionic flow on the osmotic potential and that, in turn, has led to introducing the concept of shale membrane efficiency. The membrane efficiency describes the ability of a shale to hinder ion movement when interacting with drilling fluids. If the shale completely stops ionic flow, the shale is said to act as a perfect semipermeable membrane with a membrane efficiency of unity. If the shale lets ions flow freely, the shale is said to act as a nonselective membrane with a membrane efficiency of zero.
Staverman (1952) was one of the first researchers to investigate the membrane efficiency of shale. He presented a model to estimate the reflection coefficient (i.e., the membrane efficiency) of shale membranes. He showed that the measured osmotic pressure obtained using a nonideal membrane is different from the thermodynamically predicted value. Furthermore, this measured osmotic pressure is highly dependent on the permeability of the membrane to the solutes. Following Staverman, Low, and Anderson (1958), Fritz and Marine (1983), and Ballard et al. (1994), presented theories that suggested osmosis as a mechanism for swelling pressures generated by shales. These studies all showed that a shale could act as a leaky semipermeable membrane, because it did not completely stop the flux of ions.
While the previously mentioned studies focused on verifying osmotic transport in shale and gave a qualitative measure of its membrane efficiency, these studies did not quantitatively measure the membrane efficiency of shale. Therefore, the next phase of experimental work by various researchers focused solely on quantitatively estimating the membrane efficiency of shale. van Oort et al. (1996), Ewy and Stankovich (2000), Mody et al. (2002), and Schlemmer et al. (2003) conducted pressure transmission tests to measure the membrane efficiency of shales. In addition, they studied the transport of water and ions in shales and the impact on shale stability to facilitate the improvement of water-based muds as shale drilling fluids. Their results showed that low-permeability shales could act as leaky semipermeable membranes that sustain osmotic flow of water. Also, they showed that the membrane efficiency of shales was low and ranged from 1% to 10%. They argued that shale cation exchange capacity and permeability could be responsible for the membrane behavior of shales. Based on their results, they concluded that the ability of shales to act as osmotic membranes could provide a powerful new means for stabilizing shale rocks when exposed to water-based drilling fluids.
While all these studies gave a general idea of the membrane efficiency of shale when interacting with drilling fluids, none of these studies presented a clear picture of the relationship between the membrane efficiency of shale and the properties of shale and drilling fluids. In this work, the membrane efficiency of shale, when interacting with water-based and oil-based muds, has been estimated using pressure transmission tests. The dependence of the membrane efficiency of shale on ion type and concentration in the drilling fluid is fully explored using different cations and anions over a range of concentrations. The influence of shale permeability and cation exchange capacity on the membrane efficiency is also investigated using four different shales.
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